A conventional exhausted air dispensing device for a pneumatic power nailer is disclosed in FIG. 1 and includes a cover 13 rotatably mounted to a rear end of the barrel of the power nailer and the cover 13 has a radial opening 12 defined therethrough. An end cap 10 extends from the rear end of the barrel of the power nailer and a hole 11 is defined through the end cap 10 so that the exhausted air ejects from the hole 11. A plurality of bosses 14 extend radially from a root portion of the end cap 10 so as to be received in a groove 15 defined in an inside of the cover 13. Therefore, the cover 13 can be rotated to adjust the position of the opening 12 to prevent the exhausted air together with dust or waste wooden powder from blowing to the user. Besides, because the exhausted air expelled from the holes 11 is so strong so that the cover 13 could not be securely mounted to the end cap 10 and the cover 13 could be loosened so that the position of the radial opening 12 is changed.
The present invention intends to provide an engagement structure between the cover and the end cap of the nailer, which ensures the positioning of the cover.